The present invention relates to electroluminescent displays.
Electroluminescence is the emission of light by a material when subjected to an electric field. Phosphor electroluminescence was discovered and documented in 1936. However, it was only in the 1950's that GEC and Sylvania received patents for electroluminescent powder lamps. The short lifetime, for example 500 hours, of such devices limited their usefulness.
Work carried out in the 1980's revitalised the powder electroluminescent lamp, and in 1990 the Durel Corporation demonstrated a flexible electroluminescent phosphor device that was incorporated as a backlight into a liquid crystal flat panel display. The manufacturing technique involved encapsulating the phosphor powder particles in glass beads and sandwiching the encapsulated powder between two electrodes, to which an AC voltage was applied to stimulate emission. Electroluminescent devices made according to this type of method are known as “thick film” or “powder” electroluminescent devices. This is to be contrasted with “thin film” electroluminescent devices, in the manufacture of which a thin layer of electroluminescent phosphor is deposited on a, typically glass, substrate by a method such as atomic layer epitaxy.
Traditionally, thin film technology has been used to make electroluminescent displays, and thick film technology has been used to make electroluminescent lamps, in particular backlights for liquid crystal displays (LCDs). An example of a thin film device is described in U.S. Pat. No. 5,463,279, and an example of a thick film device is described in U.S. Pat. No. 5,686,792.
A typical thick film phosphor electroluminescent device comprises a layer of electroluminescent material in a dielectric matrix, sandwiched between two planar conducting electrodes. The electroluminescent material comprises phosphor particles, typically a zinc sulphide (ZnS) powder doped with manganese (Mn), microencapsulated in a dielectric material. Typically, silver- or graphite-loaded screen-printable inks, and indium tin oxide (ITO), a transparent conductive material, respectively are used to form the electrodes on a substrate such as a polyester film. When an AC voltage is applied between the electrodes, the electroluminescent material emits light.
The inventors have recently developed thick film electroluminescent displays in which a plurality of shaped independent electrodes are provided on at least one side of a layer of shaped or unshaped electroluminescent material. A voltage may be applied selectively to each of these independent electrodes to illuminate a respective region of the display. A thick film electroluminescent display is created by selecting the configuration of the independent electrodes to represent information, for example in the form of a seven-segment display or the like.
Thus, the inventors have recently developed an addressable electroluminescent display, i.e. an electroluminescent display comprising a plurality of display areas each having the shape of a graphical element wherein each display area may be separately, selectively illuminated.
A problem associated with the manufacture of thick film electroluminescent displays is that the independent electrodes must be connected electrically to a voltage source for the display. In a convenient manufacturing technique, electrical connections are applied as conductive tracks on the rear surface of the device, for example by screen printing conductive ink. However, the tracks themselves can act as electrodes and cause the electroluminescent phosphor to emit light where the phosphor is sandwiched between a transparent front electrode and the conductive track. Thus, the conductive tracks appear as illuminated lines on the display and adversely affect the clarity of the displayed information, which is undesirable.
U.S. Pat. No. 5,686,792 relates to an electroluminescent lamp with a continuous electroluminescent dielectric layer and a patterned rear electrode overlying the electroluminescent dielectric layer. The rear electrode includes at least two conductive segments separated by a gap. An insulating layer fills the gap and a conductive interconnect overlies the insulating layer, joining the segments. The insulating layer spaces the interconnect from the electroluminescent dielectric layer a sufficient distance to reduce the electric field in the electroluminescent dielectric layer below the point at which the lamp appears luminous.
The solution to the problem of visible electrical interconnections in the context of an electroluminescent lamp provided by U.S. Pat. No. 5,686,792 has certain disadvantages. For example, the depth of the insulating layer is fixed by the manufacturing process and this depth determines a maximum voltage which can be applied to the rear electrode without causing illumination of the electrical connections in the electroluminescent display. Furthermore, the thickness of the insulating layer must be carefully controlled to ensure the invisibility of the interconnections, and this places additional constraints on the manufacturing process. Also the thickness and inflexibility of the insulating layer adds to the overall thickness of the display and detracts from its flexibility.